lecture 2

Lecture 2

Elementary Programming Primitive data types Identifiers and variables Assignment statements Arithmetic expressions

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Example Problem Statement

Compute the final grade as a weighted sum of the scores obtained in exam, lab and homework

Problem Analysis Input:

Scores for Examination, lab and homework Output:

Final grade Additional information:

Pre-determined weights for examination, lab and homework

Example: Initial Algorithm

1. Determine the weights for each assessed components. 2. Obtain the input for exam, lab and homework scores. 3. Compute the final score:

3.1 Compute the weighted exam score 3.2 Compute the weighted lab score 3.3 Compute the weighted homework score 3.4 Compute the sum of the above weighted scores

4. Output the final score.

/** * CourseGrade determines the final grade which is computed as * the weighted sum of the grades obtained in exam, lab and homework */ public class CourseGrade { public static void main(String[] args) { int examWeight = 70; // Percentage weight given to examination int labWeight = 20; // Percentage weight given to lab work int hwWeight = 10; // Percentage weight given to homework assignment double examScore; // Examination score obtained by student double labScore; // Lab score obtained by student double hwScore; // Homework score obtained by student double finalGrade; // Final grade obtained by student

// Ask student to input scores for exam, lab and nomework IO.output("Enter your exam grade: "); examScore = IO.inputDouble( ); IO.output("Enter your lab grade: "); labScore = IO.inputDouble( ); IO.output("Enter your homework grade: "); hwScore = IO.inputDouble( ); // Computer final grade as the weighted sum of exam, lab and homework scores examScore = examScore * (examWeight / 100.0); labScore = labScore * (labWeight / 100.0); hwScore = hwScore * (hwWeight / 100.0); finalGrade = examScore + labScore + hwScore; // Output the final grade IO.outputln("Your final grade is " + finalGrade); } }

Identifiers An identifier is a sequence of characters

consisted of Letters (a-z, A-Z) Underscores ( _ ) Dollar signs ( $ ) Digits (0-9, the first character cannot be a digit)

Identifiers are case-sensitive. For example: Hello, hello, WHOAMI, WhoAmI, whoami are different identifiers.

An identifier can be of any length, but cannot be one of the reserved words

(What is a reserved word?)

// valid identifiers int examWeight = 10; double examScore; double exam_Score; int perfect_score_100; // Invalid identifiers int perfect score 100; //WRONG int today2-7-2014; //WRONG int 2014y; // WRONG! int int; // WRONG!

Reserved Words

Example in our daily lives Some vehicle registration plates are

reserved for special purpose

Internet domain names, e.g. .gov, .edu

Similarly, in Java, there are some reserved words Examples of

reserved words in our daily lives

Reserved words

This list is incomplete, but covers most of the reserved words in this course

Naming Conventions

Meaningful names should be used Avoid using x, y, z, var1, var2. Uses meaningful names such as

radius, center, area, score MixedCase can be used for

lengthy variables by separating words using upper case letters Lower camelCase: starts with a

lower case letter, e.g. variables and methods

Upper CamelCase: starts with a upper case letter, e.g. classes

/* Try to avoid meaningless identifiers */ int x; double var1;

/* Meaningful identifiers with camelCase are preferrable */ double examScore; double areaOfCircle; public class BankAccount

What is a variable?

A variable is kind of identifier A variable represents a piece of

computer memory that can store a value

Typically such memory is requested by the program, and can be modified by the program

The value of a variable can be changed so that a program would be more flexible for handling different inputs

Examples: examScore, labScore, hwScore and finalGrade

A trunk of computer memory that can be used by a program to store values

A piece of computer memory

What is a variable?

T.C. Wallace Tony Leo

An analogy is the use of mailbox, each mailbox is labeled by its owner (identifier) and different kinds of mails (values) can be put into the mailbox.

How is a Variable created?

A variable is created through a declaration process A variable declaration in a program consists of

three major parts The data type, identifier and semi-colon

The data type of the variable. It has to be specified as Java has different data types

int examWeight ; The name of the variable. Also known as the identifier

A trunk of computer memory that can be used by a Java program to store values

examWeight

Initializing a variable After we have created a variable,

we need to initialize the variable (i.e. give it an initial value):

int examWeight = 70;

Declaration and initialization can be done in separate steps:

int examWeight ; examWeight = 70;

A trunk of computer memory that can be used by a Java program to store values 70

examWeight

Changing the Value of a Variable

After a variable is declared and initialized, its value can be modified by an assignment statement:

examWeight = 50;

A trunk of computer memory that can be used by a Java program to store values 70 90

examWeight

50

Retrieve a Value from a variable The current value of a variable can

be retrieved by referring to its name:

int labWeight = examWeight ;

A trunk of computer memory that can be used by a Java program to store values

examWeight

50 50 50

labWeight

Finalizing a variables value

Sometimes we want to keep a variables value unchanged throughout the program, for example: Mathematical constants, e.g. PI ( = 3.14159) examWeight, labWeight, hwWeight in the CourseGrade program

We can prevent accidental changes to the value by finalizing a variables value Adding the keyword final when declaring the variable Such variables are also referred to as constants.

We can assign value to a final variable only once


Re-assigning a final variable creates a compilation error

examWeight = 50;

A trunk of computer memory that can be used by a Java program to store values 70

examWeight

Finalizing a variables value

final

Introduction to Data Types

Data types is a very important concept in high level programming language. The way to define a variable is by specifying a

type All variables and names referred in a Java

program must be defined before it can be used. Each data type has its own properties and

memory space requirements

Jave Primitive Data Types A type defines a set of values and a set of operations

that can be applied on those values. The set of values for each type is known as the

domain for the type. Java supports eight primitive data types:

byteshort

intlong

floatdouble

char boolean

PrimitiveData Types

Java Primitive Data Types Integers

Data types byte, short, int, long

Values stored are exact without any approximation

Floating-point numbers Data types

float, double

Values stored are approximated

Booleans Data types

boolean

There are only 2 possible values: true and false

Characters Data types

char

They are used to store symbols

Data types: Integers

Name Range

byte -27 to 27-1 (-128 to 127)

short -215 to 215-1 (-32768 to 32767)

int -231 to 231-1

long -263 to 263-1

Advantages: Represents the exact numerical value without approximation Disadvantages: Cannot represent floating-point values

Data types: floating-point numbers

Name Range

float Negative range: -3.4028235E+38 to -1.4E-45 Positive range: 1.4E-45 to 3.4028235E+38

double Negative range: -1.7976931348523157E+308 to -4.9E-324 Positive range: 4.9E-324 to 1.7976931348523157E+308

(for reference ONLY, you are not required to memorize them)

The range is much larger than integers (long) (~263 only)

Expressions

In writing programs, we use expressions as building blocks, which typically describe actions the program should take

Two types of expressions Arithmetic expressions Boolean expressions

Examples: Computing final grade as a weighted sum of some

scores Check if a student received a passing grade

Arithmetic Expression

An arithmetic expression is a sequence of variables, constants, literals connected by arithmetic operators

Examples An arithmetic expression 2+3 has two literals (2 and 3)

connected by a + operator An arithmetic expression celsius*9/5+32 has one

variable (celsius), three literals (9, 5 and 32) connected by three operators (*, / and +)

Numeric literals

A literal is a constant value that appears directly in the program

There are two types of numeric literals Integer literals Floating-point literals

A compilation error occurs if the literal is too large to fit into a variable

// Integer literals byte aByte = 10; int aInt = 10;

// floating point literals float aFloat = 10.0f; double aDouble = 10.0;

// Compilation error examples float aFloat2 = 10.0; // without suffix f byte aByte2 = 1000; // too large for byte

Arithmetic operators

Some commonly used arithmetic operators: Operator Examples Result (+) Addition 2 + 3 5 (-) Subtraction 2 3 -1 (*) Multiplication 2 * 3 6 (/) Division 2 / 3 0 (Integer division) (%) Modulus

2 % 3 2 (The remainder of 2 / 3) Would work on floating point numbers too!

Example: Division and Modulus

104 3 102

34

2

b a (a/b)

(a%b)

Integer division and floating-point division

For data types storing numerical values, we have integers (int) and floating point numbers (double)

The results for integer and floating numbers are different on division: Integer division

Division between two ints: the result is the integral part of the division, the decimal part of the number would be discarded.

Examples: 2/3 results in 0, 3/2 results in 1 Floating-point division

Division between one or two doubles results in a double Examples: 2/3.0 results in 0.66666 (integer divided by double) 3.0/2 results in 1.5 (double divided by integer) 10.0/2.0 results in 5.0 (double divided by double)

Operators and Precedence Which of the following is it equivalent to mx + b?

(m * x) + b m * (x + b)

Operator precedence tells the order in which different operators in an expression are evaluated.

Standard precedence order ( ) Evaluated first, if nested then evaluate

the innermost first. * / % Evaluated second. If there are several,

then evaluate from left-to-right. + - Evaluate third. If there are several,

then evaluate from left-to-right.

Associativity Associativity is used to determine the order in

which operators with the same precedence are evaluated in an expression.

Example: 3 * 8 / 4 % 4 * 5 is the same as ( ( ( ( 3 * 8 ) / 4 ) % 4 ) * 5 ) Parentheses can be inserted if you are not sure

about (or want to enforce) the evaluation order

2 1 3 4

Assignment statements

Syntax of an assignment statement: Variable = Expression;

It means to assign the value evaluated from an expression to the variable

The original value stored in that variable (if any) will be replaced

Example: int one = 1; // variable one is initialized to 1 one = one + 1; // Now, the variable one has a value of 2

Type Conversions Type conversion changes the value of one data type to another. Type conversion can be done in two different ways: implicit and

explicit Implicit conversion is done without any special directive from the

programmer. For examples, to assign an int to a float Basic rule: The conversion is allowed if the range of values of the first

type is a subset of the second (widening conversion). Explicit conversion is done by type casting.

The name of the target type, in parentheses, is placed in front of the data type to be converted.

Example: double dValue = 100.0; int ivalue = (int) dValue; float fValue = (float) dValue;

Declaration of Data Type

examWeight

70

labWeight hwWeight labScore hwScore examScore finalGrade

20 10


int labWeight = 20;

int hwWeight = 10;

double examScore;

double labScore;

double hwScore;

double finalGrade;

? ? ? ?

Assignment of Values to Variables

examWeight

70


20 10

examScore = IO.inputDouble( ) ;

labScore = IO.inputDouble( ) ;

hwScore = IO.inputDouble( ) ;

? ? ? ?

90.0

85.0

80.5

90.0

Changing value of a variable

examWeight

70


20 10

examScore = examScore * (examWeight / 100.0 ) ;

70

0.7

90.0 85.0 80.5 90.0

63.0

?

90.0

Changing value of a variable

examWeight

70


20 10

examScore = examScore * (examWeight / 100.0 ) ;

70 85.0 80.5 63.0 ?

labScore = labScore * (labWeight / 100.0 ) ; 17.0

hwScore = hwScore * (hwWeight / 100.0 ) ; 8.05

finalGrade = examScore + labScore + hwScore ; 88.05

/** * CourseGrade determines the final grade which is computed as * the weighted sum of the grades obtained in exam, lab and homework */ public class CourseGrade { public static void main(String[] args) { final int examWeight = 70; // Percentage weight given to examination final int labWeight = 20; // Percentage weight given to lab work final int hwWeight = 10; // Percentage weight given to homework assignment double examScore; // Examination score obtained by student double labScore; // Lab score obtained by student double hwScore; // Homework score obtained by student double finalGrade; // Final grade obtained by student

Declaration of Constants

examWeight

70


20 10

final int examWeight = 70;

final int labWeight = 20;

final int hwWeight = 10;

examWeight = 50; labWeight = 40;

Simple I/O A program interacts with users through input and output devices Console output: System.out.println and System.out.print

Example: System.out.println("Hello, world!");

Console input using the Scanner class will be discussed later

I/O class for output: IO.outputln and IO.output Examples: IO.output("Enter your exam grade: "); IO.outputln("Your final grade is " + finalGrade);

I/O class for input: IO.inputInteger and IO.inputDouble Examples: double examScore = IO.inputDouble( ); int intValue = IO.inputInteger( );

Add the following statement at the beginning of your program: import comp102x.IO;

User Interface for Input

Computers are not very good in reading handwritten/printed words or understanding speech

Need user interface for inputting numbers and test to a computer Keyboard Visual input, e.g. barcode Voice input, e.g. Siri

Barcode A barcode is a machine readable representation of data Linear (or 1-D) Barcodes represent data by varying the

widths and spacings of a set of parallel lines

Developed in the 1960s and became a commercial success due to its widespread use in automated checkout systems.

Barcode Example import comp102x.IO; // This program demonstrates simple arithmetic on values represented by barcodes public class BarcodeDemo { public static void main (String[ ] args) { // Declare and initialize two long variables long value1 = IO.inputBarcode(); // Read a barcode from an existing file long value2 = IO.inputBarcode(); // Read another barcode from an existing file // Output the values represented by the two barcodes IO.outputln("The value of the 1st barcode is: " + value1); IO.outputln("The value of the 2nd barcode is: " + value2); // Add and multiply two barcodes long addResult = value1 + value2; long mulResult = value1 * value2; // Output the calculation results IO.outputln("The result of adding the two barcodes values is: " + addResult); IO.outputln("The result of multiplying the two barcodes values is: " + mulResult); // Output the calculation results as images on the file system IO.outputBarcode(addResult); }

long value1 = IO.inputInteger(); long value2 = IO.inputInteger();

Lecture 2 Example Example: Initial AlgorithmSlide Number 4Slide Number 5IdentifiersReserved WordsReserved wordsNaming ConventionsWhat is a variable?What is a variable?How is a Variable created?Slide Number 13Initializing a variableChanging the Value of a VariableRetrieve a Value from a variableFinalizing a variables valueFinalizing a variables value Introduction to Data TypesJave Primitive Data TypesJava Primitive Data TypesData types: IntegersData types: floating-point numbersExpressionsArithmetic ExpressionNumeric literalsArithmetic operators Example: Division and ModulusInteger division and floating-point division Operators and PrecedenceAssociativityAssignment statements Type ConversionsSlide Number 34Declaration of Data TypeSlide Number 36Assignment of Values to VariablesSlide Number 38Changing value of a variableChanging value of a variableSlide Number 41Declaration of ConstantsSimple I/OUser Interface for InputBarcodeBarcode Example

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lecture 2

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